Gamma-rays are widely used to produce images of extended objects, for example for medical diagnoses. When there in access to both sides of the object being studied, the conventional approach is to measure the attenuation of a gamma-ray beam passing through the object from a source on one side of the object to a detector on the other. If a wide area beam is used together with a position sensitive detector, a two dimensional map of the object is produced. To produce a 3-dimensional image, multiple two dimensional slices can be combined using computed tomography (CT) techniques. If the object being studied can be injected with a positron emitting nuclide, positron emission tomography (PET) can be used to build up a 3-dimensional image of the object by using the back-to-back 511 keV gamma-rays produced when the positron annihilates.
Throughout the specification the term gamma-ray means electromagnetic photons having an energy of about 1 keV or more and includes electromagnetic photons normally known as X-rays which range up to about 100 keV.
When there is access to only one side of the object being studied, techniques based on gamma-ray transmission are impossible. Compton scatter imaging (CSI) has been proposed as an alternative method. Gamma-rays from a source pass into the object being studied, undergo a Compton scatter back out of the object and are counted using a suitable detector. Because there is a close relationship between the angle that the gamma-ray scatters through and the energy that it loses, by measuring the energy spectrum of the scattered gamma-rays it is possible to infer the distribution of material within the object of interest. However, unfolding this distribution requires complicated mathematical deconvolution techniques. Alternatively, if a collimated gamma-ray beam is used and the direction of the scattered gamma-rays is determined, direct imaging is possible. However, such systems typically have fairly low efficiencies and scanning is required to build up a full 3-dimensional image.
If the object being studied produces gamma-rays itself (examples would include a biological specimen injected with a radiological tracer or a distant astronomical image), a 2-dimensional image of the radioactive source density can be produced using an Anger camera or a Compton telescope. The former uses a position sensitive gamma-ray detector together with a gamma-ray opaque screen with a small aperture that projects an image of the object being studied onto the detector. Large or multiple apertures can be used to increase the efficiency of the camera, but necessitate the use of mathematical deconvolution techniques to form an image. The Compton telescope makes use of the angle/energy relationship of the Compton scattering process described above to infer the direction of an incident gamma-ray by measuring its interaction with two separate position sensitive detectors. The Compton telescope can be fairly efficient, but again mathematical deconvolution is required to obtain an image.
All of these methods suffer from one or more of the following disadvantages:                Access is required to 2 or more sides of the object being studied;        Only 2-dimensional information is obtained;        The object being studied needs to contain radioactive nuclei;        Complex mathematical techniques are required to produce an image of the object;        Scanning of the object and/or source/detector are required to build up an image.        